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Add section for ZFS Special Device

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Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>
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Fabian Ebner 2019-11-07 12:06:28 +01:00 committed by Thomas Lamprecht
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@ -431,3 +431,56 @@ See the `encryptionroot`, `encryption`, `keylocation`, `keyformat` and
`keystatus` properties, the `zfs load-key`, `zfs unload-key` and `zfs
change-key` commands and the `Encryption` section from `man zfs` for more
details and advanced usage.
ZFS Special Device
~~~~~~~~~~~~~~~~~~
Since version 0.8.0 ZFS supports `special` devices. A `special` device in a
pool is used to store metadata, deduplication tables, and optionally small
file blocks.
A `special` device can improve the speed of a pool consisting of slow spinning
hard disks with a lot of changing metadata. For example workloads that involve
creating or deleting a large number of files will benefit from the presence of
a `special` device. ZFS datasets can be configured to store whole small files
on the `special` device which can further improve the performance. Use SSDs for
the `special` device.
IMPORTANT: The redundancy of the `special` device should match the one of the
pool, since the `special` device is a point of failure for the whole pool.
WARNING: Adding a `special` device to a pool cannot be undone!
.Create a pool with `special` device and RAID-1:
zpool create -f -o ashift=12 <pool> mirror <device1> <device2> special mirror <device3> <device4>
.Add a `special` device to an existing pool with RAID-1:
zpool add <pool> special mirror <device1> <device2>
ZFS datasets expose the `special_small_blocks=<size>` property. `size` can be
`0` to disable storing small file blocks on the `special` device or a power of
two in the range between `512B` to `128K`. After setting the property new file
blocks smaller than `size` will be allocated on the `special` device.
IMPORTANT: If the value for `special_small_blocks` is greater than or equal to
the `recordsize` of the dataset, *all* data will be written to the `special`
device, so be careful!
Setting the `special_small_blocks` property on a pool will change the default
value of that property for all child ZFS datasets (for example all containers
in the pool will opt in for small file blocks).
.Opt in for small file blocks pool-wide:
zfs set special_small_blocks=4K <pool>
.Opt in for small file blocks for a single dataset:
zfs set special_small_blocks=4K <pool>/<filesystem>
.Opt out from small file blocks for a single dataset:
zfs set special_small_blocks=0 <pool>/<filesystem>